This invention relates to preparation of end-capped tubular beta"-alumina ceramic bodies. In particular, this invention relates to ultrasonic end-capping of extruded beta"-alumina tubular ceramic precursors, also known as green bodies, such that after binder removal and sintering the bond is hermetic and the integrity of the bonded area is comparable to the body of the tube.
Beta"-alumina tubes need to be end-capped at one end in order to be used in construction of sodium-sulfur batteries or similar energy conversion devices. Ultrasonic end-capping as taught in this invention, can proceed by either of two routes, ultrasonic forming or ultrasonic bonding, with the latter being preferred. During the end-capping process, the ultrasonic vibrations from the ultrasonic horn are transmitted to the part and travel to the joint area of interface. Here the vibratory energy is dissipated in the form of frictional heat which melts the binder and welds the joining surfaces.
While ultrasonic bonding technique is known, it has only been used to bond materials of thermoplastic compositions. In contrast, the composition of the tubular bodies of the subject invention comprise a thermoplastic elastomer binder in combination with ceramic particulate. In view of the different characteristics of thermoplastic elastomers, it was not expected that such technique could be used to bond compositions containing these elastomers. Further, using beta"-alumina particulate precursor in the composition of the tubes, in view of its hydroscopic nature, was expected to result in an unsatisfactory bond due to the foaming conditions that were expected to develop at the joint interface of such materials during bonding. Still further, the composition of the tubular bodies, which includes waxes and oils as plasticizers, was not felt suitable for ultrasonic technique in view of the fact that these included materials usually interfere with the resins ability to transmit vibratory energy. Also, the inclusion of high levels of precursor ceramic, as in the subject composition, was thought to preclude the presence of a sufficient amount of binder material deemed necessary for successful bonding. Tubes which have been extruded using the composition of this invention had previously been end-capped using solvent bonding. However, that technique is less reliable due to such problems as bubbling of the solvent and misalignment of the cap to the tube.
This invention permits improvements in the end-capping of green body tubes comprised of a mixture of sacrificial binder, densely packed with powdered beta"-alumina precursor particulate such that after binder removal and sintering, the bond is hermetic and the integrity of the bonded area is comparable to the body of the tube. U.S. Pat. Nos. 4,158,688 and 4,158,689 to Pett et al which are commonly assigned with this application, disclose preparation of green bodies of unusual dimensional precision using sacrificial binders similar to that of this invention. In these patents, the green bodies formed include compression molded cordierite ribbed sheets and extrusion molded beta"-alumina precursor particulate tubes. The extrusion composition used to make the tubes would not be optimal for use as the composition of the tubes which are ultrasonically end-capped by the process of the subject invention.